From political parties to environmental organisations, citizens join a host of associations in order to influence policymaking and political agendas. Yet why in some western democracies do citizens join political organisations much more than in others? Drawing on a large number of crossnational surveys and data sets, Morales shows that huge crossnational variations in political membership are not so much related to social or attitudinal differences between these countries' citizens, but are explained to a great extent by the structure of the political system of each nation.